If you don't care about 100% laboratory-grade perfection, you can
reheat the gel and reform it into a block after you shoot it. This is
easily achieved by microwaving the gel and then reforming it into a block
with the normal methods.

It is suggested that you do not shoot naked gel. Use cloth or some dense
hard wood. Shooting bare gelatin will not produce enough differentiation
with your different loads. To see the difference in ammunition, some
intermediate barrier is needed. The cloth covering is much more demanding
than what you might guess.

Red meat gives the same results as shooting gel.
Trying to use rib bones will be difficult to get accurate, reproduceable
results.